L1 transposons are mobile DNA elements which are known to be active in several species including mouse and man. Their activity is known to cause disease. L1 retrotransposons move via a copy and paste mechanism and exhibit no known market integration site preferences. These features make them useful as insertional mutagens because they can tag the genes they disrupt. A insertional mutagens, L1 retrotransposons could be used to study tumor progression. Through genetic engineering they potentially could be designed to move in a tissue-specific, developmental stage- specific fashion. As such, they could revolutionize cancer genetics. However, the activity of L1 transposons in vivo is poorly understood. The frequency with which they move about is not known. It also is not well understood where and when retrotransposons are active. This proposal is designed to further our understanding of the biology of L1 retrotransposons in vivo and establish whether they can serve as insertional mutagens. To achieve these aims we will demonstrate L1 retrotransposition from integrated chromosomal sites. Next we will create a screenable marker assay for retrotransposition. This screenable marker assay will allow us to characterize the frequency of retrotransposition events without prolonged selections in antibiotics. We will use this assay to evaluate the cell types and states in which retrotransposition events are favored. We hope to address whether retrotransposition events are favored. We hope to address whether retrotransposition events occur more frequently in the transformed state and promote genetic instability in tumor cells. Finally, we will establish an in vivo model of L1 retrotransposition by adapting our L1 reporter constructs to maximize the chances of in vivo detection. We will breed our L1 transgenics to ubiquitously expressing lacZ indicator mice. Mice containing both the tagged L1 retrotransposon and the screenable marker allele will be used to eliminate the frequency of retrotransposition in vivo and determine the tissue types and developmental stages in which retrotransposition events occur.